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Classroom activity Grade 12 Engineering & Design

Vennacott Phosphorus Permit: A Nutrient Removal Charrette

Grade 12 teams rotate through three design desks, price chemical against biological phosphorus removal for a fictional plant, and defend one recommendation.

Objectives

  1. Students will calculate phosphorus loading and required removal using the pounds formula.
  2. Students will price a chemical phosphorus removal option including its added sludge disposal cost.
  3. Students will compare annualized costs of two removal strategies and defend one recommendation.

Vocabulary

Key words: total phosphorus, pounds formula, chemical precipitation, biological phosphorus removal, anaerobic selector, sludge production

Teacher overview

This is a three-desk design charrette for teams of three or four students, run once through a full rotation. Teams work as process engineers for the fictional Vennacott Water Resource Recovery Facility, which has just received a phosphorus limit it cannot currently meet. Each desk handles one piece of the decision: the permit desk converts concentrations into daily pounds, the chemical desk prices ferric chloride and the sludge it creates, and the biological desk annualizes the cost of building an anaerobic selector. One misconception worth naming before students begin is that removing phosphorus destroys it. Phosphorus is an element, so chemical precipitation only moves it out of the water and into the sludge stream, which is why the chemical option adds hauling cost that students routinely leave out of the comparison.

Activity Directions

  1. Before class, print one station card for each of the three desks and set the desks apart from one another. Each card lists only the data that desk needs, taken from the fictional Vennacott data set on the student handout.
  2. Form teams of three or four students and give each team a copy of the student handout and a calculator. Assign each team a starting desk so that the three desks are used evenly.
  3. Open with five minutes of framing. State the design problem in one sentence: the fictional plant discharges more phosphorus than its new permit allows, and the team must recommend how to close that gap.
  4. Run three 13-minute rotations so that every team visits each desk once, starting from its assigned desk. At the permit desk, teams complete handout items 1 through 4. This desk establishes how many pounds of phosphorus per day the plant must remove and what removal efficiency that represents.
  5. At the chemical desk, teams complete handout items 5 through 9. Remind teams that this desk is not finished until the added sludge has been priced, because the ferric chloride does not make the phosphorus disappear.
  6. At the biological desk, teams complete handout items 10 and 11. This desk annualizes a large construction cost over a service life so that it can be compared against a chemical bill paid every day.
  7. Bring teams back together for a ten-minute charrette. Each team posts its total annual figures for both options and states which option it recommends, naming one criterion beyond cost that supports the choice.
  8. Close the activity when every team has posted a recommendation and defended it in under one minute. Use the debrief questions in step 9 rather than a general discussion.
  9. Debrief with two specific questions. First, the chemical option costs less per year, so why might an operator still argue for the anaerobic selector? Second, the sludge line item is the one teams most often omit, so what else would that omission hide about the true cost of a chemical solution?
Vennacott Phosphorus Permit: A Nutrient Removal Charrette — Grade 12 Engineering & Design
Name: ____________________________ Date: ______________

Student Handout

The fictional Vennacott Water Resource Recovery Facility treats 2.4 MGD. Its influent total phosphorus averages 6.0 mg/L, and its new permit sets a monthly average limit of 0.8 mg/L. Use the pounds formula for every loading question: pounds per day equals concentration in milligrams per liter times flow in million gallons per day times 8.34.

The plant's two options are described below.

Chemical option: ferric chloride removes 0.20 pounds of phosphorus for every pound of ferric chloride fed. The chemical costs $0.40 per pound. Each pound of ferric chloride fed also produces 0.55 pounds of extra dry sludge solids, and sludge disposal costs $0.09 per pound of dry solids.

Biological option: an anaerobic selector retrofit costs $1,850,000 to build, has a 20-year service life, and adds $9,000 per year in power and monitoring.

  1. The plant treats 2.4 MGD with an influent total phosphorus of 6.0 mg/L. Find the influent phosphorus loading in pounds per day.
  2. The permit limit is 0.8 mg/L at the same flow of 2.4 MGD. Find the permitted effluent phosphorus loading in pounds per day.
  3. Influent phosphorus is 6.0 mg/L, the permit limit is 0.8 mg/L, and flow is 2.4 MGD. Find the pounds of phosphorus that must be removed each day.
  4. Influent phosphorus is 6.0 mg/L and the permit limit is 0.8 mg/L. Find the removal efficiency as a percent, using removal efficiency equals influent minus effluent, divided by influent, times 100.
  5. Ferric chloride removes 0.20 pounds of phosphorus per pound fed. Using a removal requirement of 104.08 pounds per day, find the ferric chloride feed rate in pounds per day.
  6. Ferric chloride costs $0.40 per pound. Using a feed rate of 520.4 pounds per day over 365 days, find the annual chemical cost.
  7. Each pound of ferric chloride produces 0.55 pounds of extra dry sludge. Using a feed rate of 520.4 pounds per day, find the extra dry sludge produced each day.
  8. Sludge disposal costs $0.09 per pound of dry solids. Using 286.2 pounds of extra dry sludge per day over 365 days, find the annual disposal cost.
  9. Using an annual chemical cost of $75,978.40 and an annual sludge disposal cost of $9,401.67, find the total annual cost of the chemical option.
  10. The anaerobic selector costs $1,850,000 to build, lasts 20 years, and adds $9,000 per year in power and monitoring. Find the annualized cost of the biological option.
  11. Using a chemical option cost of $85,380.07 per year and a biological option cost of $101,500 per year, find how many dollars per year separate the two options.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key
  1. Answer: 120.1 lb/day. Using the fictional plant's data, 6.0 mg/L times 2.4 MGD times 8.34 equals 120.096, which rounds to 120.1 pounds per day.
  2. Answer: 16.0 lb/day. 0.8 mg/L times 2.4 MGD times 8.34 equals 16.0128, which rounds to 16.0 pounds per day.
  3. Answer: 104.08 lb/day. The concentration that must be removed is 6.0 minus 0.8, or 5.2 mg/L. Then 5.2 times 2.4 times 8.34 equals 104.0832, which rounds to 104.08 pounds per day.
  4. Answer: 86.7 percent. (6.0 minus 0.8) divided by 6.0 equals 0.8667, and multiplying by 100 gives 86.67, which rounds to 86.7 percent.
  5. Answer: 520.4 lb/day. 104.08 pounds of phosphorus divided by 0.20 pounds removed per pound of ferric chloride equals 520.4 pounds per day.
  6. Answer: $75,978.40 per year. 520.4 pounds per day times $0.40 per pound equals $208.16 per day, and $208.16 times 365 days equals $75,978.40.
  7. Answer: 286.2 lb/day. 520.4 pounds per day times 0.55 pounds of sludge per pound of ferric chloride equals 286.22, which rounds to 286.2 pounds per day.
  8. Answer: $9,401.67 per year. 286.2 pounds per day times $0.09 per pound equals $25.758 per day, and $25.758 times 365 days equals $9,401.67.
  9. Answer: $85,380.07 per year. $75,978.40 in chemical cost plus $9,401.67 in disposal cost equals $85,380.07 per year.
  10. Answer: $101,500 per year. $1,850,000 divided by 20 years equals $92,500 per year of capital, and adding $9,000 per year in power and monitoring gives $101,500.
  11. Answer: $16,119.93 per year. $101,500 minus $85,380.07 equals $16,119.93, so the chemical option costs that much less each year.

Differentiation

Support

Give these teams the permit desk card with items 1 and 2 already set up as 6.0 times 2.4 times 8.34 and 0.8 times 2.4 times 8.34, so the first calculation is a keystroke rather than a formula recall. Provide a worked model of the pounds formula using a different fictional plant, such as 3.0 mg/L at 1.5 MGD, before the rotation starts.

Extension

Ask these teams to test how sensitive the recommendation is to a single assumption. Have them recalculate the chemical option's annual total if the delivered price of ferric chloride at the fictional plant rises to $0.62 per pound, then state at what chemical price the two options cost the same each year.

ELL supports

Provide a labeled process sketch showing where ferric chloride enters and where the extra sludge leaves, so the phrase "moved into the sludge" has a picture behind it. Offer sentence frames for the charrette: "Our team recommends the ___ option because it costs $___ per year," and "One criterion other than cost is ___, and it favors the ___ option."

Extensions

Suppose the fictional Vennacott facility later receives a second permit limit, this time on total nitrogen. Ask students which of the two phosphorus options would make the nitrogen limit easier to meet and why, then have them sketch the order in which the anaerobic, anoxic, and aerobic zones would have to sit for a plant chasing both nutrients at once.